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SHANGHAI, China — About 300 million people living in China’s vast countryside drink unsafe water tainted by chemicals and other contaminants, the government said Thursday in its latest acknowledgment of mounting risks from widespread pollution.

Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

The most common threat to water, after drought, is chemical pollutants and other harmful substances that contaminate drinking supplies for 190 million people, state media quoted E Jingping, a vice minister for water resources, as saying.

The report follows recent chemical spills in the northeast and south of the country that temporarily spoiled water supplies for millions of people and highlighted the severity of the pollution crisis.

The problems are not limited to the countryside. About 90 percent of China’s cities have polluted ground water, the official Xinhua News Agency reported, citing a recent nationwide survey.

In Shanghai, the country’s biggest and wealthiest city, fetid, stinky canals bubble with pollution. The city’s tap water, drawn partly from the heavily polluted Yangtze River, is yellowish and smelly, despite efforts to clean up local waterways.

Heavily polluting paper and chemical plants have long been cited as key sources of degradation of most of China’s waterways. In some areas, the problems have prompted riots by local residents outraged by chronic health problems and the destruction of their fields and fish farms.

Millions of other Chinese face risks from naturally occurring contaminants, such as excess fluorine, which affects water supplies for 63 million people, and arsenic, which taints water supplies for 2 million. Another 38 million have only brackish water to drink, the report said.

Earlier this week, authorities reported that toxins in the Bei River, in southern China’s Guangdong province, had nearly returned to safe levels after a Dec. 15 spill of more than 1,000 tons of cadmium-laced water from a smelter in the city of Shaoguan.

Cities along the Bei temporarily stopped drawing water from the river and dams were closed to keep the spill away from the provincial capital, Guangzhou.

Residents in Russia’s Far East have been warned against eating fish after a 110-mile-long slick from a chemical spill in northeastern China crossed the border earlier this week. That spill, from a Nov. 13 chemical plant explosion in the city of Jilin, forced Chinese cities along the Songhua River to shut off water for days.